Binder-free NiO/CuO hybrid structure via ULPING (Ultra-short Laser Pulse for In-situ Nanostructure Generation) technique for supercapacitor electrode

Developing a cost-effective pseudocapacitor electrode manufacturing process incorporating binder-free, green synthesis methods and single-step fabrication is crucial in advancing supercapacitor research. This study aims to address this pressing issue and contribute to the ongoing efforts in the field by introducing ULPING (Ultra-short Laser Pulse for In-situ Nanostructure Generation) technique for effective design. Laser irradiation was conducted in ambient conditions to form a CuO/NiO hybrid structure providing a synergistic contribution to the electrical behavior of the electrode. Mainly, the effects of surface morphology and electrochemical surface because of tuning laser intensity were analyzed. The samples demonstrated high oxide formation, fiber generation, excellent porosity, and ease of ion accessibility. Owing to a less than 10-min binder-free fabrication method, the electrochemical performance of the as-fabricated electrode was 25.8 mC cm−2 at a current density of 1 mA cm−2 proved to be excellent. These excellent surface properties were possible by the simple working principle of pulsed laser irradiation in ambient conditions and smart tuning of the important laser parameters. The CuO/NiO electrode demonstrates excellent conductivity and rewarding cyclic stability of 83.33% after 8000 cycles. This study demonstrates the potential of the ULPING technique as a green and simple method for fabricating high-performance pseudocapacitor electrodes.


Experimental setup and methodology
Material synthesis and preparation of electrodes. A 0.3 mm Ni sheet was acquired from Sigma Aldrich. The Ni sheet was cleaned with DI water and acetone to eliminate any residue from manufacturing and transport. Carefully handled, a commercially available high-purity conductive Cu tape was applied onto the Ni sheet, ensuring no air pockets and creases were visible. The tape-covered sheet was wiped with a Kimwipe to remove any smudges and debris. The prepared Cu-taped Ni sheet was then situated on an XYZ free-motion plate. A circular profile was designed in MarkingMate (CAD software), and the laser parameters were set on the laser software, as shown in Table 1. The setup of the laser and all the essential components such as mirrors and optical glass are best demonstrated in Fig. 1a. Upon satisfactory focal length from the preview profile, the laser irradiation was followed. A better visual is provided in Fig. 1b of the complete process. The wavelength of the picosecond was 1064 nm. The as-obtained samples were cut into smaller, rectangular pieces with the irradiated circle on one end. Similar steps were carried out for coin cell setup as previously mentioned; however, two circular coin cells of the same laser parameters were punch holed from the Cu-taped Ni sheet for symmetric electrochemical testing. The nomenclature of the samples was assigned as shown in Table 1.

Material characterization.
As for the nanoscale analysis of the morphology and structural characteristics, SEM was conducted at a magnification of × 500, × 10,000, × 35,000, and × 100,000. The surface's micro and macro-scale analysis could be possible from the SEM images. To further analyze the microscopy results in nano regime, TEM was conducted. To understand the material properties and elemental composition of the created samples, EDX was conducted. In the EDX data, the presence of oxygen would play an important role as the surface desired is to be highly oxidized. In addition to material composition, XRD was conducted to analyze the diffraction pattern, crystalline structure of the hybrid samples. Lastly, XPS was also conducted to understand the ratio of oxidation states that were present after the sample preparation.

Measurement of electrochemical properties.
A three-electrode setup was utilized as shown in Fig. 1c.
For the three-electrode configuration, the use of a Plate Material Evaluating Cell (PMEC) obtained from ALS Co., Ltd and was utilized. Two PTFE blocks sandwich the working electrode, with the top block exposing the laser-irradiated, oxidized part only. Part of the electrode sheet was exposed outside of the sandwiched block for connection to working; an Electrolyte of 0.5 M KOH was used as an ionic solution. Ag/AgCl reference electrode was acquired from ALS Co., Ltd, and an auxiliary platinum wire electrode obtained from DEK Research was used as a current collector. An SP-150 potentiostat was used to conduct electrochemical tests such as Cyclic Voltammetry (CV), Galvanostatic charge-discharge (GCD), and Electrochemical Impedance Spectroscopy (EIS) on the prepared samples. For the two-electrode setup, an MTI split test cell was used in which the coin cells were placed with a simple 2 M KOH electrolyte-soaked separator sandwiched between the two-coin cells. Simple tests  www.nature.com/scientificreports/ such as CV, GCD, and cycle retention were conducted to examine the electrochemical behavior of the symmetric cell. The calculation of specific capacity was done using Eq. (1) for three-electrode setup. Here, J is the current density which can be calculated by dividing area, A from current, i, and t is the discharge time.

Results and discussion
Morphology and structural properties. Upon visual inspection of the laser-treated samples, a relationship was observed. As the average power was increased, oxidation and mass ablation were evident. For P8, the Cu on the Cu tape was oxidized, demonstrating a fragile film of CuO present on the substrate. For P12, the Cu tape was largely oxidized and converted into a thin film with a bit of oxidized Ni which was visible. Going into the higher intensity, visually, dark oxidized color with a hint of copper was noticed, indicating the presence of both NiO and CuO. SEM was conducted under the 4 samples to find the topology, porosity, surface area, and presence of nanostructures. Starting with P8, the higher magnification shown in Fig. 2a demonstrated microstructure with granules attached to the surface and no presence of nanostructures. In higher magnification, however, the surface was more uniformly distributed and showed excellent porosity as demonstrated in Fig. 2e. The lower intensity of the laser beam allowed less pulse energy to be transferred to the substrate promoting rapid quenching due to the presence of a large temperature gradient in ambient conditions. Progressing to P12, P15, and P20, as seen in Fig. 2b-d, the nanofibrous growth was more evident. P12 and P20 demonstrated the most nanoscale fiber growth of the hybrid electrode, with P12 demonstrating fibrous growth throughout. The lower magnification of the microscopic image shows more considerable granules growth as the power is increased. This is due to the agglomeration effects of reaching near plasma temperature 43,44 , as seen in Fig. 2f-h. As the macroporous structural properties also dictate the better rate capability of a pseudocapacitors cell, P12 demonstrated far better uniformity in surface distribution, porosity, and better surface area compared to P15 and P20. For a rough estimation of the porosity levels, ImageJ was utilized by adjusting the threshold of the SEM images. The porosity levels for P12, P15, and P12 were 40%, 36% and 21% respectively. Overall, P12 demonstrates all the required surface properties ideal for design 45 . We determined the specific surface area of the fiber through ImageJ analysis, as presented in Table 2. The fiber dimensions were obtained from TEM images, assuming the  www.nature.com/scientificreports/ fibers to be cylindrical in shape 37 . Although there may be some margin of error due to these assumptions, the values in Table 2 provide a clear indication of the trend in specific surface area, which aligns with changes observed in both material and electrochemical properties. EDX was conducted on the basis to discover the elemental composition to prove the presence of oxygen due to the oxidation process via laser irradiation as shown in Fig. 3a. P8 shows low oxygen detection and high copper presence with no traces of Ni. However, the 12W power sample, P12, demonstrated a high oxygen presence along with Ni and Cu, indicating the formation of a hybrid structure from the irradiation. The high-power samples, P15 and P20, showed similar results to P12; however, the presence of oxygen wasn't as dominant as in P12. A closer look at the P12 sample with SEM shows that the morphology inhabits granular and fiber growth on the surface through × 10,000 magnification. The fiber is grown in a granular shape and exhibits an urchin-like morphology shown in Fig. 3b. This morphology follows the microporous structure requirements essential for enhanced surface area for maximum charge storage and provision for maximum redox-active sites. At × 100,000 magnification demonstrated in Fig. 3c, the morphology confirms the presence of nanoscale growth of the active material, proving the ULPING technique is successful and efficient for the electrode fabrication method.
X-ray diffraction pattern were analyzed for analysis of crystalline properties and the overall structure of the samples. With the increase in power, the collective contribution of Ni and Cu are observed in the hybrid structure from the diffraction peaks. The two common phases of copper oxide, Cuprous oxide, Cu 2 O and Cupric Oxide (CuO) were discovered. In addition, diffraction patterns of plain Cu were also highlighted in the survey. Both crystal system of cubic and monoclinic were observed from both copper oxide phases. Figure 3d highlights the index reference to each corresponding angle as referred to the JCPDS card No for Cu 2 O and CuO (05-0667 and 45-0937 respectively). As the laser power is increased, the inclusion of cubic NiO phase becomes more apparent. This aligns with the XRD pattern of standard cubic NiO crystalline structure [46][47][48][49][50] . TEM was conducted on all samples; the results of TEM closely conform to the findings in SEM. The improvement in surface area can be assessed by the fiber diameter. P8 shows fiber strand diameter of 70 nm obtained by taking the average as depicted in Fig. 3e. P12 on the other hand showed remarkable interlinked fiber with strand diameter being roughly 15 to 20 nm in size as can be noticed in Fig. 3f. This proves 12 watts being the optimal power setting for maximum fiber growth for hybrid structure. Figure 3g of P15 sample shows no significant fiber interlinked, however, maximum oxidation can be assumed due to particle agglomeration. Additionally, SEM images showed minimal fiber growth in P15 which agrees with TEM findings. Lastly, P20 showed the next best nano regime particle or fiber size. By averaging the fiber diameter, 25 nm is ideal assumption for P20 as shown in Fig. 3h. www.nature.com/scientificreports/ XPS was conducted on all four samples. Figure 4a illustrates a survey of all the species present on the surface layer via a complete XPS spectrum. The peaks of Cu 2p 3/2 , Ni 2p 3/2, and O 1 s were dominant and each was deconvoluted into many signals at higher resolution spectra to find surface oxidation. The XPS spectra of core level Cu 2p present satellite structure in the higher energy range indicating the presence of the CuO phase. This satellite shake-up structure was present in all samples except for P8. The Cu 2p 3/2 was deconvoluted into three signals as shown in Fig. 4b of which the ~ 932.7 eV was ascribed to Cu + , ~ 933.6 was ascribed to Cu 2+ and the third peak was ascribed to ~ 935.6 denoting copper hydroxide (Cu(OH) 2 ) species on the surface layer. From the XPS data of Cu 2p 3/2 core levels, as the energy density of the laser to carry out irradiation increased, the Cu 2+ phase was less dominant and the Cu + phase became more apparent on the surface layer. Observing the Ni XPS high-resolution spectrum presented in Fig. 4c, The Ni 2p 3/2 peak was analyzed to find the Ni oxidation states. Like Cu 2p 3/2 , Ni 2p 3/2 demonstrated a shakeup satellite structure at higher energy levels. The Ni 2p 3/2 peaks were deconvoluted into two signals of which the peak at ~ 854.2 eV was ascribed to the Ni 2+ phase of NiO and the second signal at ~ 855.6 eV was ascribed to the Ni 3+ state of Ni 2 O 3 . Going from P12 to P20, it was observed that increasing energy density allowed for an increase in the Ni 3+ phase compared to the ratio of the Ni 2+ phase. Finally, the XPS peak of O 1 s core level was analyzed. Here, the peaks were deconvoluted into four sub-peaks. The signals were identified as shown in Fig. 4d. The intense signal of peak 1 is ascribed to lattice oxygen, O L at ~ 529.6 eV responsible for the bonding of O-Ni or O-Cu. Peak 2 at ~ 531.2 eV is ascribed to oxygen vacancy or mental deficiency. Peak 3 at ~ 531.8 eV ascribes to O-H group bonding and finally, peak 4 at ~ 533 represents the adsorbed O 2 elements. In the case of all the samples, the lattice oxygen, O L bonding environment is the most dominant compared to oxygen vacancies, O V . On all, the data demonstrated in the XPS conforms with the findings of EDX and SEM. T proves the effects of pulse energy, fluence, and laser intensity, a survey of all four samples are presented in Fig. 4e in which as the energy intensity of the laser beam increases, the peak of Ni becomes more apparent. Great energy intensity allows for deeper penetration of the sample with CuO being successfully fused onto the NiO structure and abundant ionization with surrounding O 2 to form a hybrid structure. P8 demonstrated no traces of Ni since the lower energy intensity of the beam was equipped which was not able to penetrate through the Cu layer and ablate the Ni surface 51-59 .

Electrochemical properties
Electrochemical tests were conducted to confirm the finding of structural characterization results, and observations were made. A series of tests such as CV, GCD, and EIS was conducted to analyze the electrochemical behavior of the laser-fabricated electrode via our ULPING technique.
CV was conducted on the sample to understand the charge storage mechanism of the fabricated electrode. A series of scan rates were performed on all the samples, of which 50 mV s −1 is presented in Fig. 5a. The stable, chosen potential for CV testing was 0 to 0.6 V in 0.5 M KOH, where no gas evolution reactions were visible. Unique redox reactions were observed as the power samples varied. P8 sample to which thin CuO film was generated due to the lowest pulse energy presented a non-uniform CV curve to which the redox peaks were not visible. However, due to the non-rectangular CV shape, pseudocapacitance was confirmed. The 8W power sample demonstrated the least charge storage and minimal rate capability due to the absence of nanoscale structures www.nature.com/scientificreports/ on the surface, unlike the high-power samples. The unique CV of the lower power samples conforms with the findings of EDX, in which the detection of Cu was significant; however, Ni was least observed. This is due to the pulse energy of the laser being relatively low, therefore not allowing the pulses to penetrate through the CuO film. The redox peak, however, was sharply dominant beyond the P8 sample. P12 demonstrated the maximum charge storage among all samples due to the optimal pulse energy for pulses to penetrate through the CuO. The Cu tape on the 12W sample was oxidized and held onto the surface of the Ni substrate, and the Ni was also oxidized under the Cu tape. As a result, the dominant curve from CuO and NiO redox is visible. A complete composite fusion process is visible in P15 and P20 samples in which the CuO nanoparticles are deposited onto the NiO. P12, however, demonstrated to exhibit the best performance. A series of scan rates were conducted on P12, as shown in Fig. 5b. An increase in the capacitive behavior was visible as the scan rate increased. Since the samples portrayed nanoscale structural formation and enhanced large surface area, the combinational effect of charge storage due to faradaic reactions and non-faradaic, surface charge accumulation defines the pseudo-capacitor equal working mechanism. To understand the charge storage mechanism of the fabricated samples, Dunn's method was employed as denoted by the power law equation presented in Eqs. (2) and (3)  Here, i is the peak current from anodic curves, a and b are adjustable variables and v is the scan rate. The b-value is calculated from the slope of log (i) vs log (scan rate) for multiple data sets of peak current at various scan rates. A b-value close to 0.5 governs a diffusion-controlled working mechanism whereas a b-value closer to 1 exhibits capacitive or surface-controlled charge storage. From this analysis, as shown in Fig. 5c, the b-value of P12 was 0.86 which is ideal for pseudo-capacitance. To further quantify the contribution of the charge storage mechanism a series of CVs at slow scan rates were conducted from which Eq. (4) can be applied to separate the capacitive and diffusion behavior in the CV profile. Here the summation of k 1 v and k 2 v 1/2 defines the capacitive and diffusion-controlled behavior to make up the current as a function of voltage.
Rearranging Eq. (4) yields, www.nature.com/scientificreports/ By plotting I(V)/v 1/2 vs. v 1/2 , a best-fit line connects the data points from which values of k 1 and k 2 are representative of slope and y-intercept respectively. Carrying this process at various potentials provides a separation profile for both capacitive and diffusion processes. In Fig. 5d, the diffusion process is separated from the CV curve of P12 at 2 mV s −1 . Once known of k 1 and k 2 value areas are, an i d can be easily obtained by multiplying the desired scan rate. In Fig. 5e, as the scan rate increases, the capacitive nature becomes more apparent 62,63 .
For calculating the ECSA or the electrochemical surface area of P12, a capacitive dominant narrow window was analyzed from the previous CV curve to be 0.0 V to 0.2 V. Multiple CV scan rates from 0.001 V s −1 to 1 V s −1 were conducted as shown in Fig. 5f. The resultant change between anodic and cathodic current density was found for each scan rate and the graph was plotted as demonstrated in Fig. 5g. A line of best fit provides the slope which represents the double-layer capacitance or C dl which was 9.03 mF for P12. Comparing this with the second-best performer in this work P20, the C dl for P20 was only 2.71 mF. Assuming the C sp to be 0.040 mF cm −2 which is typical for NiO, the ECSA of P12 is 225.75 cm 2 as compared to 67.75 cm 2 of P20 [64][65][66] .
GCD tests of all samples were conducted at 2 mA cm −2 as presented in Fig. 5h. P12 demonstrates superior performance as compared with other samples. This was once again possible due to many reasons given above such as excellent surface area, porosity, and double-layer interfacial charge storage all contributing to the excellent performance of P12.
With all the tests, it was obvious to understand that the laser intensity and fluence have a direct relationship with each other as well as a close relationship with the morphology in creating a hybrid structure. Too much pulse energy assures excellent formation of hybrid structure, but at a loss of surface area. These effects mainly occur due to the aftereffects of pulse bombardment at the local spot. Due to high thermal diffusivity because of dense pulse intensity, the melt pool temperatures exceed the spot size which promotes granule agglomerates as well as the evaporation of nanofiber 67 .
To prove the synergistic effects taking effect in the P12 sample, a copper and nickel sheet were irradiated with the same parameter settings used for P12 using the ULPING technique (Refer to Table 1). The thickness of the sheet and the supplier of the sample were all the same and claimed 99% purity of material. For these samples, CV and GCD were run on the samples. The results are demonstrated in Fig. 6a and b. Figure 6a shows CuO with minimal redox peaks, however having a horn-like structure at 0.5-0.6 V. Similarly, the nickel oxide CV profile demonstrates faradaic behaviour with prominent redox peaks. The CV profile of P12 demonstrates collective contribution in which the horn-like profile is seen at 0.5-0.6 V, similar to CuO and the reversible redox peaks from NiO. GCD was also conducted on the same samples. The P12 samples has a large depletion time of charge as seen in Fig. 6b. This proves that the hybrid structure provides collective contribution in proving better performance.
P12 exhibits superior performance compared to its constituents (CuO and NiO) and other samples due to a variety of factors. In comparison to the individual metal oxides, the Cu 2+ and Ni 2+ ions present in the CuO-NiO nanoparticles possess partially-filled 3d-orbitals, which facilitate more efficient engagement in redox processes. This enhanced redox activity contributes to the superior performance of P12. According to various studies reported in the literature, an optimal Ni to Cu ratio of 1:1 is critical for achieving optimal electrochemical performance 30,68 . P12 exhibits superior performance due to the optimal Ni:Cu ratio obtained through 12W laser irradiation, as evidenced by the EDX data presented in Table 3 and Fig. 6c. The ratio of Ni:Cu is determined by the fluence and pulse intensity delivered to the substrate via the laser, as well as the material characteristics that are altered in response to the supplied heat. Furthermore, as confirmed by TEM and SEM analyses, P12 exhibits an enhanced surface area with an excellent nano fiber network, resulting in increased exposure to active surface redox sites at the interface 32,69,70 . This characteristic further contributes to the superior performance of P12.
EIS was conducted to find the electrochemical behavior of the samples in AC analysis. The frequency range was picked from 100 MHz to 100 mHz, and an equivalent circuit was designed that best relates to the fabricated setup. The equivalent open circuit is a traditional Randles circuit with a series connection of constant phase elements followed at the end. The Nyquist plot is shown in Fig. 7a. The higher frequencies demonstrate all the electrochemical resistance, such as series and charge transfer resistance, followed by the diffusion of ions into www.nature.com/scientificreports/ the electrode presented in the mid-frequency range. At lower frequencies, interfacial charge storage is analyzed. P8 had high internal resistance of both series and charge transfer presented by the depressed semi-circle and ion-diffusion slope at mid-frequency. Past the semi-circle, P8 showed a semi-infinite medium diffusion or the Warburg impedance with a 45°-line exhibiting battery-like behavior. The high impedance or poor conductivity is due to the inferior surface area required for better rate capability of the pseudocapacitor electrode and muchneeded EDL formation for capacitive contribution.
On the other hand, the higher power samples such as P12, P15, and P20 were relatively closest to the finite space, demonstrating solid capacitive characteristics. P12, amongst all the other samples in lower frequency, demonstrates excellent capacitive behavior due to its near 90°. The line at lower frequencies. From Fig. 7b a closer look at the internal and diffuse layer resistance can be analyzed; P15 exhibits the lowest internal resistance. This is due to the successful laser pulse ionization fusion of the CuO onto the NiO, proving excellent for better electrode and electrolyte conductivity at the interface. In addition, the surface area at both nano and macro scales was excellent, proving best in internal resistance. P8, despite having the highest internal resistance, managed to have lower electrode resistance. This is likely due to the uniform porous structure of the electrode. P12 and P20 demonstrated the highest internal resistance. This is obvious since, as mentioned earlier, in the P12 sample, the welded CuO tape had a satisfactory connection to the oxidized NiO substrate, hence the higher internal resistance. However, the surface area for this sample proved excellent, which assisted in the formation of the EDL, as seen by the higher slope in mid-frequency, indicating the presence of EDL. P20, on the other hand, also demonstrated high internal resistance majority due to poor surface area on the macroscale due to increased particle agglomeration and fiber evaporation at high temperatures from larger laser pulse energy. Overall, P12 demonstrated excellent capacitive characteristics and was further analyzed by simulating with the equivalent open circuit as shown in Fig. 7c. The fit was made as shown in Fig. 7c, and various internal resistance was found as shown in Table 4.
The bode plot was analyzed to understand the phase angles of the components of the designed equivalent circuit with varying frequencies seen in Fig. 7d. From the phase angle analysis, it is observed that the constant www.nature.com/scientificreports/ phase of 45° at lower frequencies of P8 was dominant, proving the findings from the Nyquist analysis. This also demonstrates the Warburg impedance at the lower frequencies for P8. P15 and P20 at lower frequencies demonstrate stability at a phase angle around the high 60 s, progressing toward zero phase angle with a positive slope at mid frequencies, demonstrating the ion diffusion process. P12 was most promising to exhibit capacitive behavior at − 78°, which is very close to − 90° from the ideal capacitor, therefore, proving an excellent candidate for a pseudocapacitive device 71 . Figure 7e. Investigates the samples with impedance because of varying frequency. At lower frequencies, the device's impedance is more significant; however, as the frequency has increased, the impedance has dropped significantly 72 . Of all the samples, P12 demonstrates the most negligible impedance compared to other samples.
To further study the conductivity of the electrode samples, four-point probe was utilized. The measurement of resistivity is shown in Table 5. The resistivity of the hybrid structure is more than that of pure Ni and Cu. The conductivity is measured by taking the reciprocal of the resistivity data acquired.
Cycle retention, efficiency, and capacity. Capacity retention and columbic efficiency were analyzed with high current density charge-discharge of the P12 sample for 8000 cycles since it demonstrated the best electrical properties. A current density of 40 mA cm −2 was applied to the electrode resulting in rapid charge and discharge. The capacity retention of P12 was roughly 83% after 8000 cycles, which proved to be promising, as shown in Fig. 8a. The coulombic efficiency also proved excellent, maintaining a 97-98% efficiency during charge and discharge. The loss in capacity retention was investigated by conducting SEM on the pre-cycle and post-8000 cycle of the P12 sample surface morphology. Comparing Fig. 8b and c, it was easy to conclude that the drop in performance was due to a loss in nanofibers which reduce the specific surface area of the sample. Another reason www.nature.com/scientificreports/ for the drop in performance would be due to the battery material transition metals, which witness permanent phase over time transition and growing impedance due to poor rate capability. However, considering a less than 10-min fabrication, the cost of the electrodes could be extremely affordable and mass scalable due to rapid manufacturing. XPS was conducted on the samples after 8000 cycles of charge discharge. Figure 8d-f demonstrates Cu 2p 3/2 , Ni 2p 3/2, and O 1 s core levels before cycling. Here all the peaks are similar to as described in the structural characterization section of this manuscript. However, Fig. 7g-i demonstrates the deconvolution of the signals after 8000 cycles at discharge states. Both Cu and Ni show an increase in the ratio of Cu(II) Hydroxide at 934. 75 and NiOOH at ~ 855 after the reduction reaction due to discharging state. The O 1 s peak demonstrates a drop in lattice oxygen and increases in the O-H bonding group. The areal capacitance was calculated using discharge current densities. A specific capacitance of 25.8 mC cm −2 was achievable at a current density of 1 mA cm −2 , proving excellent performance. However, the specific capacity decreases as the scan rate is increased as seen in Fig. 9a. This is evident from many works of literature published, as this is most commonly due to the insufficient time for ions to diffuse into the electrode surface. A coin cell setup was proposed with a symmetric assembly of the P12 electrodes with a separator soaked in a high concentration 2 M KOH electrolyte. A series of CV, GCD, and capacity tests were conducted.
Presented in Fig. 9b is the CV for the P12 coin cell at a scan speed of 10 mV s −1 and 100 mV s −1 . The suitable potential window was chosen to be 1.2 V, demonstrating maximum stability, dominant redox peaks, and perfect reversibility. The expanded voltage provided symmetric anodic and cathodic peaks, which were desirable for symmetric cells. GCD tests were also conducted on the coin cells as shown in Fig. 9c. The areal capacitance was found to be 16 mF cm −2 at a 2 mA cm −2 scan rate exhibiting excellent charge storage characteristics. The discharge capacity of the coin cell was monitored by the cycling of GCD at 5 mA cm −2 for 3000 cycles seen in Fig. 9d. Interesting results were obtained, demonstrating an increase in the discharge capacity of the symmetric cell of P12. The capacity was almost doubled until 2500 cycles, after which the capacity decreased. Many works of literature that made use of laser for the formation of electrodes, such as laser deposition, have reported an increase in retention due to the activation of the active material over long cycling, which could be due to the surface area modification over time [73][74][75] . These retention results are significantly different as compared to the CD test using the three-electrode setup. However, the two-electrode setup is based on the most realistic model of the design.
The as-synthesized sample, which was irradiated with 12W of laser energy, exhibited the formation of a hybrid NiO/CuO structure. The hybrid structure displayed synergistic effects and improved performance in the electrochemical behavior, as compared to single-phase NiO or CuO. This is a remarkable achievement, as it was achieved within a 2-step process that took less than 10 min for a 1 cm 2 active oxide surface. This fabrication process is extremely cost-effective, as well as a chemical-free, green synthesis approach. In the current state of the art of supercapacitor technology, many research works demonstrate remarkable electrochemical results of the supercapacitor; however, the fabrication practices are not practical, as some research reports a fabrication time of 24 + hours with a multi-step process. The novelty of this work lies in its simplicity of combining two www.nature.com/scientificreports/ earth-abundant transition metals to form a hybrid structure with the help of a pulsed laser, which has not yet been reported in the literature.

Conclusion
In this work, the ULPING method was presented to develop a hybrid structure for hybrid structure. Pulse ionization fusion of high purity Cu tape onto Ni sheet to form a CuO/NiO hybrid structure for electrode was possible via picosecond laser. Mainly analysis of morphology with a variation of laser intensity or pulse energy was demonstrated. This method created self-standing, binder-free 3D nanostructures which exhibit remarkable performance as a candidate for the pseudocapacitive electrode. At 12W average power, the best electrode characteristics were observed, such as excellent synergistic effects, great surface area in both micro and nanoscale, uniform surface area distribution, high porosity, superior conductivity, and excellent reversibility of the reactions. An areal capacitance of 25.8 mC cm −2 was achievable at a current density of 1 mA cm −2 proving to be excellent considering the fabrication took less than 10 min following a simple 2-step process. The capacity retention of 83% was achievable after 8000 cycles of charge-discharge. The good retention well compensates for the affordable cell that could be manufactured at an economical and scalable method using the approach used in this work. This work promotes the idea of green synthesis and scalable fabrication of electrodes with the ULPING method.

Data availability
The datasets used and/or analyzed during the current study are included in the published article. The raw data used are available upon request from the corresponding author.